1. Field of the Invention
The present invention relates to a switching element utilizing an organic thin films formed of an organic material, an organic electroluminescence (hereinafter also called xe2x80x9cELxe2x80x9d) element including a light emitting layer formed of an organic EL material based thin film utilizing the electroluminescence of an organic compound material which emits light by injection of electrons or holes, and an organic EL display device having a plurality of the switching elements arranged in matrix.
2. Description of the Related Art
An electroluminescence element display composed of a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as a display which provides for low power consumption, high display quality, and reduced thickness. As illustrated in FIG. 1, each organic EL element 200 includes a transparent substrate 1 such as a glass substrate or the like having formed thereon a transparent electrode 201 formed, for example, of indium tin oxide (ITO). At least one organic material layer 202 comprising an electron transport layer, a light emitting layer, a hole transport layer and so on, and a metal electrode layer 203 are laminated on the transparent electrode 201. The transparent electrode 201 serving as an anode is applied with a plus voltage, while the metal electrode 203 serving as a cathode is applied with a minus voltage. i.e., a direct current is applied across the transparent electrode and the metal electrode, to cause the light emitting layer within the organic material layer 202 to emit light.
In the organic EL element, excitors are generated by recombination of the electrons injected from the metal cathode and the holes injected into the light emitting layer from the transparent anode, and the excitors emit light in the course of radiation and deactivation. Thus, the organic EL element 200 may be electrically represented as an equivalent circuit as illustrated in FIG. 2. As can be seen from the figure, the element can be replaced with a circuit configuration composed of a capacitive component C and a light emitting diode component E, having a non-symmetric conductivity, coupled in parallel with the capacitive component. Thus, the organic EL element can be regarded as a capacitive light-emitting element. As the organic EL element is applied with a direct current light-emission driving voltage across the electrodes, a charge is accumulated in the capacitive element C. Subsequently, when the applied voltage exceeds a barrier voltage or a light emission threshold voltage inherent to the element, a current begins flowing from one electrode (on the anode side of the diode component E) to the organic functional layer which supports the light emitting layer so that light is emitted therefrom at an intensity proportional to this current. The Voltage V-Current I-Luminance L characteristic of such an element is similar to the characteristic of a diode. Specifically, the current I is extremely small at a light emission threshold Vth or lower, and abruptly increases as the voltage increases to the light emission threshold Vth or higher. The current I is substantially proportional to the luminance L. The organic EL element as mentioned, when applied with a driving voltage exceeding the light emission threshold Vth, exhibits a light emission luminance in proportion to a current corresponding to the applied driving voltage. On the other hand, the light emission luminance remains equal to zero when the driving voltage applied to the element is at the light emission threshold Vth or lower which does not cause the driving current to flow into the light emitting layer.
An organic EL element display device is a light emitting device which has an image display array formed of a plurality of light emitting pixels, i.e., organic EL elements arranged at respective intersections of rows and columns, i.e., arranged in matrix. An exemplary method of driving an organic EL element display device is called a simple matrix driving mode. A display device in accordance with the simple matrix driving mode has a plurality of anode lines and cathode lines arranged in matrix, sand an organic EL element is connected to each of intersections of the anode lines and the cathode lines arranged in matrix. Either of the anode lines or cathode lines are sequentially selected and scanned at regular time intervals, and the other lines are driven by a driving source in synchronism with the scanning, thereby causing organic EL elements at arbitrary intersections to emit light. In this mode, since each organic EL element is lit for an accessed time, a larger current and a higher voltage are required for a larger display screen.
For providing a larger screen size for a display device, organic EL display devices driven in an active matrix driving mode are contemplated in addition to those of the simple matrix driving mode. The display device of this mode is such one that replaces the anode lines and cathode lines with scanning signal lines and data signal lines, respectively, and thin film transistors (TFT) are used for switching elements arranged at respective intersections. Respective pixels are applied with currents by switching associated thin film transistors, causing organic EL elements to emit light. For TFT, an element made of p-Si, a-Si can be employed. Alternatively, MOS-FET (Metal Oxide Semiconductor Field Effect Transistor) may be used to form TFT.
For example, with MOS-FET as a switching element, two reverse conducting regions are formed on a semiconductor substrate, for example, a Si substrate. A silicon oxide (SiO2) thin film and a metal gate electrode are sequentially deposited on the surface of the substrate between the inverse conductive regions. The conductivity on the surface of the substrate is controlled by an electric field applied from the metal gate. Therefore, a Si wafer is required for a display substrate, and a semiconductor substrate is required for a polysilicon substrate and so on. In addition, since an inorganic material must be deposited on such substrates, high temperature processes are typically used for its manufacturing.
Display devices having a larger screen are much in demand. However, if inorganic material switching elements, which require high-temperature processes in the fabrication, are employed for a large organic EL element display device of the active matrix driving mode, the resulting display device will be inevitably expensive.
It is therefore an object of the present invention to provide an organic thin film switching element which can be fabricated at relatively low temperatures, and too provide an organic EL element display device which has the organic thin film switching element formed on a common substrate.
An organic thin film switching element according to the present invention comprises:
an insulative film;
an organic thin film made of an organic material, the insulative film and the organic thin film being laminated one over the other;
a pair of opposing gate electrodes sandwiching a laminate composed of the insulative film and the organic thin film; and
an intermediate electrode disposed between the organic thin film and the insulative film.
In one aspect of the organic thin film switching element of the present invention, the organic material is an insulating organic compound.
In another aspect of the organic thin film switching element of the present invention, the organic material is a material having an electron transport property. Alternatively, the organic material may be a material having a hole transport property. 5. In addition, the organic material may have a hole and electron transport property.
In still another aspect of the organic thin film switching element of the present invention, the intermediate electrode comprises a pair of opposing electrodes spaced apart from each other, the opposing electrodes being positioned on an interface between the organic thin film and the insulative film between the gate electrodes.
An organic electroluminescence element display device according to the present invention having a display array formed of a plurality of light emitting sections, which comprises:
a substrate having a plurality of first display electrodes formed on a surface in correspondence to the light emitting sections;
an organic material layer formed on each of the first display electrodes and including at least one organic electroluminescence material layer capable of emitting light by injecting electrons or holes thereinto;
a second display electrode formed in common on the organic material layer; and
an organic thin film switching element formed on the substrate and connected to at least one of the first and second display electrodes, and including; an insulative film; an organic thin film made of an organic material, the insulative film and the organic thin film being laminated one over the other; a pair of opposing gate electrodes sandwiching a laminate composed of the insulative film and the organic thin film; and an intermediate electrode disposed between the organic thin film and the insulative film.
In one aspect of the organic electroluminescence element display device according to the present invention, the organic thin film made of an organic material of the organic thin film switching element is formed of a portion of the organic material layer.
In another aspect of the organic electroluminescence element display device according to the present invention, the light emitting sections are arranged in matrix.
In another aspect of the organic electroluminescence element display device according to the present invention, the display device further comprises a capacitor formed on the substrate, and connected to at least one of the first and second display electrodes and the organic thin film switching element.
In further another aspect of the organic electroluminescence element display device according to the present invention, the substrate and the first display electrode are transparent.
In still another aspect of the organic electroluminescence element display device according to the present invention, the intermediate electrode of the organic thin film switching element comprises a pair of opposing electrodes spaced apart from each other, the opposing electrodes being positioned on an interface between the organic thin film and the insulative film between the gate electrodes.
In still further another aspect of the present invention, the intermediate electrode and the gate electrode for injecting carriers of the organic thin film switching element are made of a material having a high work function in a case that the carriers are holes. In this aspect of the present invention, the intermediate electrode of the organic thin film switching element comprises a laminate including a first layer made of a material having a work function substantially equal to that of the organic thin film, and a second layer made of a material having a work function lower than that of the first layer.
In still another aspect of the present invention, the intermediate electrode and the gate electrode for injecting carriers of the organic thin film switching element are made of a material having a low work function in a case that the carriers are electrons. In this aspect of the present invention, the intermediate electrode of the organic thin film switching element comprises a laminate including a first layer made of a material having a work function substantially equal to that of the organic thin film, and a second layer made of a material having a work function higher than that of the first layer.